1. Field of the Invention
The present invention relates in general to methods of directing spacecraft payloads and in particular to a method and apparatus for determining and correcting for the pointing error of a phased array antenna on a spacecraft.
2. Description of the Related Art
Satellite systems are widely used to transmit information to many ground users. In satellite-based communication, it is desirable to transmit information to ground-based users in certain areas, but not the ground-based users in other areas. This is accomplished with the use of xe2x80x9cspot beamsxe2x80x9d that concentrate the energy of the transmitted signal to a limited terrestrial area. To assure optimum reception by all ground-based users, to prevent interference among users in different areas, and to reduce the probability of unauthorized reception at ground stations not authorized to receive the transmitted spot beam, it is important that the spot beam be accurately directed to the proper terrestrial locations. Deviation of antenna pointing typically causes a drop of signal power for communications to and from the spacecraft and ground user in the satellite""s services areas, thus degrading the communications services provided by the satellite.
Antenna pointing is usually controlled by a control system so that antenna communication beams will be accurately directed to the proper target(s).
Spot beam pointing accuracy can be limited by many factors. One of these factors is deformation of the structures supporting the phased array antenna on the spacecraft bus/body. Such errors can result from thermal gradients, launch environment effects, or other factors. Further, because sensors that are used to determine spacecraft pointing are usually placed at locations remote from the transmitting or receiving antennas and the components subject to structural deformation, such errors are typically unobservable by these sensors.
One technique for ameliorating this problem is to use an attitude sensor such as a star tracker, Earth sensor, or beacon sensor very close to or on the communication antenna itself. Unfortunately, this approach cannot be economically applied to satellites that have multiple communication antennas. Also, the use of beacon sensors can be unacceptably expensive because a terrestrial beacon station must be maintained for the on-board beacon sensor. This is especially the case for non-geosynchronous satellites because a single terrestrial beacon station will not be able to cover the entire orbit of the satellite and many stations are usually needed. What is needed is a system and method for compensating for these errors. The present invention satisfies that need.
To address the requirements described above, the present invention discloses a method and apparatus for determining pointing of a phased array. The method comprises the steps of receiving a signal from each of a plurality of signal sources at at least one receiving sensor disposed away from the phased array in a direction at least partially toward a receiver of a transmitted signal from the phased array, and determining the phased array pointing from the received signals. The apparatus comprises a receiving sensor for receiving a signal from each of a plurality of signal sources, the receiving sensor disposed away from the phased array in a direction at least partially toward a receiver of a transmitted signal from the phased array, and an array pointing computer for determining the direction of the phased array from the received signals.